1. Field of the Invention
The present invention relates to hardmetals, specifically cemented tungsten carbides, which are produced by powder metallurgical procedures, including liquid phase sintering, and are typically comprised of a refractory metal carbide, principally tungsten carbide, but possibly including carbides of tantalum, titanium, niobium, columbium or others, and a binder metal, generally cobalt or nickel or a combination thereof. Hardmetals of this type are composites, and have been produced commercially for at least 60 years, having application as cutting tool materials, mining tools, dies and punches of all sorts, and wear parts.
2. Description of the Prior Art
Properties of cemented carbides range from high hardness (high wear resistance) to high toughness (high strength), but may also include excellent corrosion or oxidation resistance and resistance to galling in certain applications or to welding to the work material (cratering) in other applications. These properties are primarily controlled by the composition of the hardmetal and by the size and shape of the metal carbides. The amount or proportion of binder metal plays an extremely important role in determining hardmetal properties; low levels of binder create a hardmetal exhibiting high hardness and high levels of binder create a hardmetal exhibiting more toughness or strength. Improvement in either of these properties is generally accompanied by a decrease in one or more of the other properties.
Binder metal content may vary from as low as 1% by wt. of the hardmetal composition to as high as 25% by wt. Cobalt is the predominant binder metal, but nickel may be used either by itself or in combination with cobalt to provide improved corrosion resistance in certain applications.
Scientists and metallurgists have long sought ways to improve one or more properties of hardmetals without an accompanying decrease in other properties. For instance, it has long been known that the properties of hardmetals can be improved by joining them with other materials to create composite materials. For example, the strength of a hardmetal product can be improved by clamping or brazing or even by casting the hardmetal onto or into a relatively tough base material such as steel. In so doing, the wear resistance (hardness) property of the hardmetal is not compromised, while the toughness (strength) of the new composite product is improved. Brazed hardmetal on steel plates or steel tool holders are typical applications for such composites.
Conversely, the hardness or wear resistance property of a cemented carbide hardmetal can be improved by coating the hardmetal with an even harder material such as TiN, Al.sub.2 O.sub.3, TiC, or other materials. In such products the relatively high strength property of the hardmetal (relative to the coating) is not compromised while the wear resistance of the new composite product is improved.
In the products discussed above, the composites are comprised of dissimilar materials. It is often desirable to form these composites of similar materials. Use of similar materials in the composite may provide superior toughness, wear resistance, or corrosion resistance properties without sacrificing any of the other properties within the hardmetal article itself.
Very few composites formed from similar materials are known. One such composite is disclosed in U.S. Pat. No. 4,722,405 to Langford in which two different grades of hardmetal compositions are compacted together in a manner in which one-half of the article is comprised of a first grade of cemented carbide composition and the other half is comprised of a second grade of cemented carbide composition. When sintered, this composite of composites may be comprised of two distinctively different hardmetal compositions, and the line of demarcation between them may be horizontal or vertical. However, Langford does not provide a composite in which the entire surface or selected surfaces are different in composition and properties from the core of the article.
Fischer et al., U.S. Pat. No. 4,743,515, discloses a hardmetal product having a property gradient from its surface to the core of the article. Specifically, the hardmetal product has a relatively high hardness at the surface and a relatively high toughness at the core. This result is created by controlling the cobalt binder content of the product such that the cobalt content is relatively low at the surface with progressively higher binder content toward the center or core.
In the Fischer et al. composite, the control of the binder content is provided by means of a diffusion or migration process which causes the concentration of binder metal to be lower at the surface than in the core, thereby creating the property differential. This composite cannot provide a product having distinctively different cemented carbide grade compositions, grain sizes or alloy contents in selected surfaces or on all surfaces as needed for certain applications.
Jacobs et al, U.S. Pat. No. 4,956,012, discloses a composite in which nodules of one grade of hardmetal composition are dispersed uniformly in a matrix of another grade of hardmetal composition. There is no gradient in property or composition from surface to core in such a composite. Moreover, such a composite is unable to produce articles having distinctly different compositions in selected surfaces since the Dispersion Alloyed Composite Carbide disclosed therein is a uniform mixture throughout the composite.
Drake, U.S. Pat. No. 4,398,952, describes a powder metallurgical article having a continuous mechanical property gradient from core to surface. This gradient is produced by a powder metals production technique which entails a continuous change of composition of the powder metal from core to surface or vice versa. The properties of an article produced by this method have a continuous gradient from core to surface rather than having a distinct separation in grade between core and surface or selected surfaces. In addition, the method disclosed in Drake for producing such a composite is cumbersome and expensive and is not suitable for carbide component production.
Steigelman et al., U.S. Pat. No. 4,003,716, discloses a tape cast cemented refractory metal carbide having improved sintered density. In Steigelman et al., a cemented carbide article is produced by tape casting a slurry, forming the cast tape into a desired shape and firing the tape. If one attempted to apply the flexible product from this invention to a sintered or unsintered product, problems would occur. For instance, difficulties arise in bonding the tape cast to a core, especially where the core has not been previously sintered. Accordingly, the production of a layered composite carbide product has been difficult to accomplish.